The present invention relates to temperature compensation and, more particularly, to a dynamic temperature compensation method for a turbidity sensor used in an appliance for washing articles.
Reducing the amount of energy consumption in appliances or machines for washing articles, such as dishwashers or clothes washers, is a significant problem, in part because a large amount of energy is needed to heat the liquid, such as water, for washing such articles. Thus, decreased liquid consumption for such machines may result in significant improvements in energy efficiency. Several techniques are available to indirectly monitor cleanliness of the articles, including a device for measuring or sensing the turbidity of the liquid used to wash the articles.
Turbidity sensors that employ an electromagnetic radiation source, such as a light emitting diode (LED) for emitting electromagnetic radiation which propagates within the cleansing liquid, typically suffer from temperature variation effects, such as power output variation as a function of temperature. The temperature variations effects, if left uncorrected, can substantially degrade the accuracy of the turbidity sensor. For example, an LED having a temperature coefficient of about 4,000 parts per million (ppm) per .degree.C can result in unacceptable accuracy over the temperature range of operation of the turbidity sensor. U.S. patent application Ser. No. [388,383] (RD-24,017) entitled "Temperature Compensation Method For A Turbidity Sensor Used In An Appliance For Washing Articles", by E. Berkcan, assigned to the same assignee of the present invention, filed concurrently herewith and herein incorporated by reference, provides a temperature compensation method which advantageously allows for adjusting any turbidity measurements or values obtained during cleansing operations. The adjustment is based on factory-determined temperature compensation parameters, such as the temperature coefficient of the LED. These factory-determined parameters allow for determining the response of the LED, and, in turn, the response of the turbidity sensor as a function of temperature, at least over a desired temperature range. Although the above-referred application allows for substantially reducing turbidity sensor errors due to temperature variation effects in the LED, it will be appreciated that the factory-determined compensation parameters are generally derived from the temperature response of randomly selected LED samples which may somewhat deviate from the actual response of any specific LED. Typically, such factory-derived parameters are then stored in a memory module while the appliance is being assembled in the factory. Once the appliance is deployed in the field such factory-derived parameters remain fixed in the memory module and thus the temperature compensation capability may be somewhat reduced if the temperature response of the sensor changes in the field due to aging and other conditions, such as environmental conditions and/or field replacement of the sensor with a modified LED model. Thus, it is desirable to provide a temperature compensation method based on compensation parameters which can be dynamically derived even after the appliance is deployed in the field, i.e., outside the factory. It is further desirable to provide a dynamic temperature compensation method based on the specific temperature response of the actual LED used in any given appliance.